WAAS Messages

The Wide Area Augmentation System (WAAS) is the United States Satellite Based Augmentation System. The programme, started in 1992, is being carried out by the Federal Aviation Agency (FAA)[1] and is specially developed for the civil aviation community.[2] The system, which was declared operational in late 2003,[3] currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.[4] WAAS service area includes CONUS, Alaska, Canada and Mexico.[5] The WAAS programme is continuously in evolution; two development phases have been already covered, a third is in progress, and there are plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.[6]

WAAS Messages

Every satellite-based wide area augmentation system (SBAS), as the Wide Area Augmentation System WAAS, does provide ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:

GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.

Wide Area Differential (WAD): differential corrections to the existing GPS and GEO navigation services computed in a wide area to improve navigation services performance.

Satellite orbit and clock corrections to the existing satellite navigation services (GPS and GEO), as well as the estimation of errors associated to satellites (UDRE).

Ionospheric corrections for a given grid of points, as well as the estimation of errors associated to ionosphere (GIVE).

Tropospheric corrections. Satellite orbit/clock corrections and ionospheric corrections are dynamically modelled. The SBAS shall communicate the user the corrections that are available to be used by the receiver. The information of the models is packed on messages to be sent to the user. On the other hand, tropospheric corrections are statically modelled, which means that corrections are tabulated and the information does not depend on any external behaviour but the user position (a mean troposphere is assumed). The algorithm for computing the tropospheric correction is available to the global community (section A.4.2.4 of MOPS).[7]

In addition to this, navigation data for each GEO satellite supporting ranging service is also transmitted through SBAS.
SBAS interacts with the user via the Signal in Space (SiS). The way the SBAS delivers to the user the aforementioned corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) is through messages encoded in the signal. These messages are sent each second with a data rate of 250 bits.

For a detailed description of SBAS Messages, see article EGNOS Messages; this article is fully applicable to WAAS, taking into account the following differences:

The standard gives to the SBAS system the possibility to use Message Type 27 (SBAS Service Message) or Message Type 28 (Clock-Ephemeris Covariance Matrix Message), but not both, as a way to control a potential degradation of the service in different regions. Whilst EGNOS has chosen to use Message Type 27 for this purpose, WAAS broadcasts Message Type 28.